1. Field of the Invention
The present invention relates to a mask such as a photomask or a reticle used for manufacturing a semiconductor device.
2. Description of the Related Art
In an integrated circuit semiconductor device such as a dynamic random access memory (DRAM) device, the integration has been developed, so that the width of lines of patterns in a photolithography process has become very fine.
In a prior art photolithography process, if a photomask has a line-and-space pattern and an isolated pattern, the width of each line of the line-and-space pattern is the same as that of the isolated pattern. As a result, a difference in width of images of lines is generated between the line-and-space pattern and the isolated pattern, which is called an optical proximity effect. This will be explained later in detail.
In another prior art photolithography process for a photoresist layer formed on a semiconductor substrate, even if the thickness of the photoresist layer is uneven, a photomask is formed by patterns having the same line width. As a result, a difference in width of images of lines is generated between regions where photoresist layers having different thickness are formed, which is called a standing wave effect. This will be explained later in detail.
In the prior art, the optical proximity effect and the standing wave effect are both diminished by a mask bias method which applies bias to patterns.
In the mask bias method, however, it is difficult to manufacture a photomask having patterns including optimum bias amounts, particularly, when the patterns are more refined.